88 ANNUAL OF SCIENTIFIC DISCOVERT. 



a current of water, and, after replacing, repeat this several times 

 on the first day, and on the following days only once or twice. 

 Sometimes — when the basic-iron compound has commenced to 

 make its appearance, but only if the tank be sufficiently deep to 

 allow the deposit to remain undisturbed — the bubbles may be 

 removed by slightly tapping the plate. 



The concentration of the bath is not of primary importance, but 

 one consisting of sal-ammoniac 3 pounds, copperas 4 pounds, and 

 water 30 pounds, has been found practically a good one. Since a 

 deposit also forms in absence of the ammonia, it is proved that it 

 is not composed of nitride of iron, though it be much more readily 

 formed in presence of the salt. 



To obtain a good polish, the coating when completed is rubbed 

 with a little oil, which must again be removed, always entirely by 

 polishing. When thin the lower surface is quite smooth, but with 

 increasing depth it assumes a granular, or somewhat velvety ap- 

 pearance. A deposit 2 millimetres in depth may be easily ob- 

 tained in a fortnight. — Dingier' s Polyt. Jour., il., January, 1868. 



Some recent experiments by M.i^lein, of St. Petersburgh, are 

 thus alluded to in the " Scientific American" : — 



*' His starting-point was the known process of covering engraved 

 copper plates with a coating of steel, which is quite successful in 

 a bath composed of the chlorides of ammonium and iron, to which 

 he added a minimum quantity of glycerine. Nevertheless, all 

 who have attempted coating with steel must have observed, when 

 endeavoring to give greater thickness to a very thin and brilliant 

 layer of steel, that the surface cracks, and the deposit detaches 

 itself from the cathode in very brittle spangles. Other baths, 

 composed in a uniform manner, and capable of being employed 

 under the same conditions, must therefore be used. They may be 

 classed under two categories, comprising baths composed of sul- 

 phate of iron, and sulphate or chloride of ammonium. The first 

 bath consisted of a solution of the double salt sulphate of iron and 

 sulphate of ammonium ; the second was composed of an admix- 

 ture of the concentrated solutions of each of these two salts, in 

 the proportions of their equivalents ; the third bath, which dis- 

 tinguished itself meritoriously from the others, was obtained by 

 taking a solution of sulphate of iron, precipitating the iron by 

 carbonate of ammonium, and dissolving the precipitate in sul- 

 phuric acid, thus avoiding all excess of acid. For the preparation 

 of the baths in the second category, he either mixed solutions of 

 chloride of ammonium and sulphate of iron in the proportions of 

 their equivalents, or dissolved in a solution of sulphate of iron, at 

 a temperature of about 15° Reaumur, as much chloride of ammo- 

 nium as it would take. All these baths were as highl}' concen- 

 trated and as neutral as possible. For an anode, plates of sheet 

 iron were used, presenting a surface nearly 8 times as large as 

 that of the copper cathode. Upon the employment for decompo- 

 sition of one of Daniell's cells, there were formed upon all the 

 cathodes, in the course of 24 hours, irregular deposits full of 

 cracks, which, on the slightest attempt to remove them, broke 

 into a thousand pieces. 



